<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        input {
            color: #999;
            height: 50px;
            width: 300px;
            font-size: larger;
        }
    </style>
</head>

<body>
    <input type="text" value="手机">
    <script>

        /* 
            需求：
                1. 获取焦点，如果value是默认值("手机")，清空value，字体颜色加深。
                2. 失去焦点，如果是空，则添加默认值，字体颜色变淡。
        */
        let input = document.querySelector('input');

        input.onfocus = function () {
            if (this.value == "手机") {
                this.value = "";
            }
            this.style.color = '#333';
        }

        input.onblur = function () {
            if (input.value == "") {
                this.value = "手机";
            }
            input.style.color = "#999"
        }
    </script>

</body>

</html>